This invention relates, generally, to optical fiber couplings and, more specifically, to optical fiber splices between double clad and single mode optical fibers.
In optical fiber applications, and particularly optical fiber telecommunications applications, it is often necessary to couple two optical fibers together by splicing. In some cases, the splice is between a double clad fiber having a pump inner cladding and a single mode fiber. An example is the coupling of the output of a double clad fiber laser to a single mode fiber. Such a system is shown in FIG. 1.
In FIG. 1, a laser diode pump source 14 is coupled to a double clad fiber laser 16. The fiber laser 16 includes a double clad fiber (DCF) 18 fiber that receives pump light in its inner cladding from diode 14. The double clad fiber 18 is coupled to a single mode fiber (SMF) 26, such as a Flexcore(trademark) 1060 fiber. Appropriate fiber Bragg gratings 10 and 12 are provided to form a fiber laser cavity, as shown, with grating 12 being formed in the single mode fiber. The DCF is coupled to the SMF via a splice that, as is typical, is located at the output end of the fiber laser just before the output coupler (ie., the grating 12). The location of the splice in FIG. 1 is identified by broken line box 24. Notably, a similar splice connection would exist if the double clad fiber were part of a fiber amplifier rather than a fiber laser. In such a case, gratings 10, 12 could be omitted, although a grating at the location of grating 10 might be used for stabilizing the pump diode source. Use of such a stabilizing grating is taught in U.S. Pat. Nos. 5,485,481 and 5,715,263, which are incorporated herein by reference.
Generally, it is not practical to make the DCF sufficiently long that all the pump light provided from the pump source is absorbed in the fiber laser. Depending upon the design of the fiber laser, its length, the amount of pump source power provided and the operating wavelength of the fiber laser, several watts of residual pump light may be present in the inner pump cladding of the DCF at the point of the DCF/SMF splice. Due to the difference in the diameter between the DCF and the SMF, a majority of the excess pump light escapes at the splice from the inner cladding. This is shown in FIG. 2, which is a schematic view of splice 24. In another arrangement, the cladding of the SMF may be about the same size as the inner cladding of the DCF, in which case the light from the inner cladding of the DCF would be transmitted through the SMF cladding, and easily coupled into the buffer material.
As shown in FIG. 2, although a portion 20 of the residual pump light in DCF 18 scatters out of the tapered portion of the inner cladding and into the surrounding environment, another portion 22 of the residual pump light is guided along the SMF cladding. Indeed, a significant portion of the residual pump light is coupled into the cladding of the SMF and is guided by the cladding until it reaches buffer 28. Buffer 28 is an outer covering of the SMF, and may comprises, e.g., an acrylate. The buffer may be either an original covering of the SMF, or possibly a covering added following the splicing of the two fibers. Under certain circumstances, the power level of residual light portion 22 can be sufficient to incinerate or burn the buffer and cause the fiber laser to fail catastrophically. Such a failure usually does not occur immediately, but may develop over a period of 15 minutes to several days of continuous operation. Ultimately, of course, failure depends upon the amount of residual pump power 20 being guided into the SMF cladding.
Prior art DCF/SMF splice packages exist that attempt to address the problem of excess residual pump light reaching the SMF cladding, and an example of such a package is shown in FIG. 2A. This package is intended for a DCF/SMF fusion splice, and has scattering means formed on the exposed surface of the cladding of the SMF. This scattering means is in the form of residual pump light scattering centers. In this embodiment, the scattering centers are epoxy beads 30, which have a refractive index higher than the refractive index of the SMF cladding 32. As a result, the light is scattered out of the SMF cladding through the respective light scattering centers. The DCF/SMF fusion splice 34 is housed within housing 36, which may be a glass tube, or a half cylindrical portion of a tube, with the ends of the tube sealed with UV cured epoxy joints 38, which form rigid end portions. This entire structure is covered by an external housing (not shown) that provides protection from the environment and from handling. The external housing may comprise a metal tube of a material such as stainless steel or passivated copper, and the tube covers the plastic buffer end portions 40, 42 of each of the fibers 44, 46, respectively. The ends of the metal tube are sealed with a soft adhesive material such as silicone.
While the foregoing package structure helps to scatter some of the residual pump light, it does not efficiently scatter all the light and dispose of this light in an efficient and securable manner. Also, manufacture of this package structure is time consuming, particularly the placement of the scattering centers.
In accordance with the present invention, an optical fiber splice apparatus is am provided that supports a splice between two optical fibers, typically dissimilar fibers, while coupling residual pump energy away from the buffer materials of the fibers. After the fibers are spliced together, a covering is provided that surrounds at least an outermost light-guiding portion of a first of the fibers, and preferably surrounds adjoining portions of each of the fibers. The covering is preferably an optical epoxy, and has a refractive index that is higher than the refractive index of an outermost cladding of each fiber. As such, residual pump energy in the fiber cladding is conducted out into the covering material and away from the fiber splice.
In a preferred embodiment, the covering material is surrounded by a glass capillary that is rigid enough to provide rigid support to the splice. The capillary is substantially transparent to the residual pump light so that it may be conducted from the covering to the capillary, and preferably out an end of the capillary. In one embodiment, the capillary is itself surrounded by a rigid housing that provides strong support to all the components inside. Materials for the housing may include stainless steel or passivated copper. The capillary may be secured to the housing by an adhesive epoxy, preferably one that is also substantially transparent to the residual pump light. In this way, pump light in the capillary may be conducted into the adhesive epoxy. In one particular embodiment, a surface of the housing facing the capillary is light absorbent at wavelengths within the wavelength range of the residual pump light such that light passing through the capillary and the adhesive epoxy may be absorbed at the housing surface.
The housing surrounding the splice apparatus of the present invention may be part of an optical gain device, such as a fiber laser or a fiber optic amplifier. The splice may be a coupling between a double-clad fiber of a fiber laser and a single-mode fiber receiving the optical energy output from the laser. As residual pump energy in the double-clad fiber reaches the splice, it exits the fiber cladding into the covering material. It is then coupled into a capillary that surrounds the covering and that is also located within the device housing. Much of the pump light travels through the capillary to its end, where it exits. Some of the pump light is coupled from the capillary to the adhesive material securing the capillary to the housing, which is also transparent to the residual light. This light may also be conducted out the end of the adhesive, or may reach an inner surface of the housing, where it is absorbed.